REE mineralization related to weathering of the late Permian Emeishan basalts

Abstract

Paleo-weathering crust rare earth element (REE) sedimentary ores (PWREOs) (on-site weathering profiles (paleosols) and allogene deposits (claystones)) related to weathering of the Permian Emeishan basalts in western Guizhou Province, Southwest China, were selected to advance our understanding of the controls on mobilization and redistribution of the REEs during basalt weathering, which is crucial for efficient exploration for this type of deposits.REE concentrations in the paleosols ranged from 88.24 mg/kg to 1684.21 mg/kg (mean value 582.01 mg/kg). The entire paleosol profiles are enriched in light REEs (LREEs) relative to heavy REEs (HREEs). Intense weathering of the parent Emeishan basalts formed two enrichment zones hosted mainly within the BC horizon adjacent to the lower iron-pan horizon and near the BC-C horizon interface of the paleosol profiles. High‑titanium Emeishan basalts with a relatively high initial REE concentration (271.20 ± 94.29 mg/kg) is the basis for REE enrichment in paleosols. The kaolinite adsorption is crucial for REE immobilization in paleosol profiles. The paleosol profile developed in a flat landscape with good drainage conditions (e.g., ridgetop) (profile WNEM07 in this study) has a higher ∑REEs (reaching 1684.21 mg/kg), with mean values of LREEs/HREEs and (La/Yb)N up to 7.83 and 39.56. HREEs might preferentially migrate vertically and laterally in groundwater, resulting in the enrichment of those elements in the paleosol profile at footslope (profile WNEM06 in this study) and in the REE enrichment zone that near the BC-C horizon interface of the paleosol profiles. Kaolinite combined with REEs in the weathering products was continuously transported to the sediment basin by flowing water, thus causing further enrichment of REEs in claystones, ranging in concentrations from 89.06 mg/kg to 9964.95 mg/kg (mean 1151.39 mg/kg). The formation of PWREOs may include three stages, i.e., the formation of the Emeishan basalts with high initial REE contents during the magmatism of the Emeishan Large Igneous Province (ELIP), the primary enrichment of REEs in the weathering process of the Emeishan basalts, and the further enrichment of REEs in weathering product transportation and sedimentation.